This invention relates generally to the field of electrostatic printing and more particularly to an improved non-mechanical printer for use in an electrographic printing process.
Mechanical printers are not suitable for printing at high speeds, e.g., more than 10,000 lines per minute. Instead, in connection with high speed printing, a number of processes employing non-mechanical printing, such as, for example, electrostatic printing, have been developed.
In the field of electrostatic printing it is desirable to distinguish between two processes: the electro-photographic process and the electrographic process. In the electro-photographic printing or recording process, a layer of photo-conductive material is uniformly charged with a positive charge by means of a corona discharge. The character or the like which is to be printed is then optically projected onto the photo-conductive material, as a result of which the material becomes conductive at the illuminated points and the previously existing positive charge disappears. The print is rendered visual with a colored powder (the toner) which is positively charged and which adheres to the discharged points, but is repelled by the non-illuminated, still-charged points of the data carrier.
In the electrographic process a data carrier composed of an insulating top layer and a conductive carrier layer is printed upon. During the printing process a charge image corresponding to the characters which are to be printed is applied to the data carrier by a corona discharge or the like. In this instance the charge of the insulating top layer may be positive, in which event the charge image is visualized by a negatively charged toner.
Hitherto electrographic printing processes have employed only coated paper, which is inevitably more expensive than uncoated, so-called normal paper. On the other hand, increased costs of material compel use of the cheapest possible paper in high speed printers.
Transfer printing processes can be used for printing on normal paper. In this process the data carrier is in the form of an intermediate carrier, e.g., a drum upon which the charge images are produced. After the charge images have been developed on the intermediate carrier, they are transferred to the normal paper and fixed. A so-called recording head is used to apply the charge images to the data carrier and can be of such known construction as, for example, an electrode comb.
In electrography it is important that the distance between the recording head and the data carrier be as short as possible. In addition the distance must be maintained as constant as possible. In previously known processes the special paper has been brought in contact with the electrographic recording head, as a consequence of which a minimum distance between the recording head and the data carrier has been produced. If, on the other hand, the transfer process is used, it is not possible to achieve such a small and constant distance between the recording head and the data carrier as a consequence of the lack of roundness of the intermediate carrier (for example, a drum), and as a result of the wear due to the contact between the recording head and the intermediate carrier.
An object of the present invention involves the provision of a non-mechanical printer in which the distance between the recording head and the data carrier is small and is maintained at a constant value. In accordance with the principles of the present invention this object is realized by virtue of a cushion of gas which is produced on the surface of the data carrier, the recording head floating on said cushion. With the aid of the gas flowing out of nozzles this cushion of gas between the recording head and the data carrier can be produced such that the distance between the data carrier and the recording head is approximately 10 .mu.m.
Additional details and features of the present invention, along with other objects and advantages thereof, will be readily apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawing, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts disclosed herein.